It is known that spacecraft, aircraft and satellites must accurately determine absolute orientation (i.e., roll, pitch and yaw) to realign their onboard attitude control system (ACS) and correct for instrument drift and buildup of errors affecting accurate and precise attitude determination. Attitude control is of particular importance in aircraft to maintain a stable operating environment, in surveillance satellites to track another object in space, and in remote imaging satellites to provide precise earth imaging and reconnaissance.
In the western hemisphere, weather forecasting methods rely heavily on data supplied by the Geostationary Operational Environmental Satellites (GOES series), operated by the National Oceanic and Atmospheric Administration (NOAA). The GOES series was developed from the prototype “advanced technology systems” 1 and 3 (ATS-1,-3) launched in 1966 and 1967, respectively. These and all subsequent systems have been implemented with scanning imaging systems that are able to produce full disk images of the earth at one km resolution for visible channel and four km resolution for infrared channels in about 20-30 minutes.
Prior methods have been developed to perform ACS functions. One such method, for example, measures angular positions of stars and compares those measurements to known quantities. In U.S. Pat. No. 5,963,166, Kamel teaches providing a spacecraft camera image navigation and registration (INR) system to point a satellite and camera. Therein an onboard computer performs calculations based upon information from a star tracker, gyro, and earth based sensor data to determine the attitude of the satellite. Kamel employs the use of star tracker equipment to detect stars in the ever-changing area above the satellite. The star tracker compares detected star positions with a star table and then determines vehicle attitude from the detected star positions. Star trackers provide attitude accuracy on the order of 5-20 microradians. The star tracker equipment required to detect the faint star light and maintain the star tables may be costly.
In Ring U.S. Pat. No. 5,959,576 there is disclosed a satellite attitude determination system using a global positioning system (“GPS”) and line of sight communication instead of star tracker equipment. Ring provides a dual access pointing laser receiver on one satellite and laser transmitters on other satellites to determine relative azimuth and elevation. The relative orientation is combined with global navigation satellite systems (“GNSS”) position data to determine the attitude of the satellite. Ring provides attitude determination for a communication satellite without the use of star tracker equipment. However, Ring's design concept provides coarse attitude accuracy using radar signals not suitable for applications requiring greater accuracy, such as remote imaging or surveillance satellites. Although Ring provides accurate attitude determination employing laser communication links, it potentially represents a very complex and costly system.
In Van Dusseldorp U.S. Pat. No. 5,943,008 exemplifies an attitude determining system utilizing a GPS. According to Van Dusseldorp, at least three sets of signals are respectively received from three antennas on board a vehicle. Each signal is received in a separate time domain slot, with each signal respectively receiving information from a respective satellite and a separate dedicated channel.
However, it is found that multiple antenna GPS systems provide relatively coarse attitude accuracy on the order of 1 mille radiant at best and therefore may not be used in applications requiring greater accuracy, such as in remote imaging satellites or surveillance satellites.
Techniques such as these and existing interferometric GPS attitude determination methods employing multiple antennas are complex, expensive and provide only coarse information with milliradian accuracy.
The newest of the GOES satellites (GOES-E located at 75° west longitude and GOES-W located at 135° west longitude) are 3-axis stabilized and configured to observe the earth with one panchromatic visible and four infrared imaging systems per satellite. The visible imaging systems use a “flying spot” scanning technique when a mirror moving in two axes East-West and North-South scans a small vertically oriented element of the fully viewable scene (the instrument's full area of regard) across an array of eight vertically arranged silicon pixels. The individual pixel field of view is about 30 microradians. Each scene element is sampled for just under 50 microseconds in order to support this slow scanning method. The GOES satellites payload stability must be extraordinarily high so that almost no relative motion occurs between any one scan line of the samples. Accordingly, the payload pointing does not nominally deviate further than one-third of a pixel during an entire one-second duration scan. Since there are over 1,300 scan lines to create a full disk image, it takes about 22 minutes to create the full image. Operationally a full disk sampling technique is actually done once every three hours to allow more frequent coverage of an entire visible hemisphere rather than a more frequent sampling of smaller regions.
During normal operation, GOES series satellites provide gray-scale and infrared images of different portions of the earth at between 5, 15 and 30-minute intervals. Limited regions may be sampled as frequently as about once per minute during “super rapid scan operations” (SRSO). In practice, SRSO operations are rarely used because coverage of other areas is too important to be neglected for long periods of time. Moreover, significant earth based events that occur during lapses in coverage of a particular region may be missed. In other words, the satellite's sensor may be looking at an uneventful portion of the earth's surface when the significant activity is occurring at another portion of the earth's surface. Furthermore, phenomena that may occur a night can only be seen in the infrared channels which have a much coarser spatial resolution than the visible channel and otherwise are subject to the same limitations that are inherent in a scanning system.
GOES satellites provide a system that is optimized for monitoring cloud motion, but is far less suitable for observing other GEO physical events. At visible wavelengths, clouds are efficient diffuse mirrors of solar radiation and therefore appear white with variations of brightness seen as shades of gray. Color, enhancing the contrast and visibility of the earth's surface background, may actually detract from cloud visibility in a scene. Moreover, adding color may triple the amount of information and thus the size of a digitized image, which creates a burden on the transmission demands for the broadcast portion of the satellite system. Furthermore, observations of significant but perhaps transient phenomena that occur in time scales of seconds or minutes (such as volcanoes, lightening strikes or meteors) may be late or not observed at all. Accordingly, the information provided from systems such as the GOES system, although reliable, would, if enhanced, be able to provide a high resolution “watchdog” service that reliably reports real-time information over a significant portion of the earth's surface. Also, in other systems for example, “video” style loops created from successive images having relatively coarse temporal resolution may lack the continuity needed to provide truly reliable information if cloud movements between image samples are much greater than a pixel dimension. The temporal coherence among the pixels of a scanned image and between the co-registered pixels of successive images will degrade as the time required to create the image and the elapsed time interval between scans increases. These effects may have adverse impact on the fidelity of any “image” created to represent the state of the earth at a given moment, but particularly to attempts to build animations using successive co-registered scanned images of a given area.
Various other weather satellites, in addition to the GOES satellites, include a Japanese weather satellite, MTSAT-1R located at about 140° east longitude exhibiting a coverage area that covers the Southeast Asia and Australian areas of the world. The Chinese FY (Feng-Yang) satellite is located at 105° east longitude and shows a substantially overlapping coverage with the MTSAT-1R satellite. The METEOSAT series of the European space agency with METERSAT-8 located near 0° east longitude and requires a license to decrypt and thus limits distribution for three days after observation. In contrast, the GOES, MTSAT-1R and FY satellites have open reception and distribution via NASA funded Internet links. Other satellites that perform similar operation include the Indian INSAT ID, INSAT-2 series, and KALPANA which are located near 74° and 93.5° east longitude, and the Russian system, GOMS/ELECTRO, which is located near 77° east longitude and has been out of service since September 1998. A common feature of these different systems is that they employ a spin stabilized or 3-axis stabilized satellites and imaging systems that require 20 minutes or longer to acquire a full disk image of the earth. Furthermore, the systems use the long scan period to provide a variety of spatial resolutions but all of which are coarser than 1 km at the Nadir point.
There have been a number of proposals made in the past by various individuals and groups to place a camera on a large commercial communication satellite positioned in GEO. In each case, the camera would operate as a parasitic device, in that the camera would use the power and communication subsystem of the satellite to support its operational requirements. The most recent and detailed proposals were made by Hughes Information Technology Corporation, a former subsidiary of Hughes Aircraft Company and the MITRE Corporation. These proposals are discussed below.
The Hughes proposal was described under various names such as “EarthCam”, “StormCam” and “GEM” (GEOstationary Earth Monitor) and involved a television style imaging system using a two dimensional charge coupled device (CCD) detector array to create an image of 756 pixels wide by 484 pixels high at intervals that range from between two minutes to eight minutes. The frame rate for this TV-style camera was determined by compression limitations in the satellite's meager 1-5 Kbps housekeeping data channel capacity. The Hughes proposal described placing a digital camera on board one or more of Hughes' commercial telecommunication satellites (COMSAT). This parasitic camera was to operate using power provided by the COMSAT and deliver data to a Hughes ground operation center by way of a very low data rate housekeeping telemetry link. Data was then to be distributed to various users from this single command and control facility.
The system proposed employing cameras placed on board the Hughes satellites to be located at 71° west, 101° west, 30° east and 305° east longitude. Upon receipt and after processing, data would be distributed via landline or communication satellite links to end users. The single visible imaging system would operate with a zoom mode so as to achieve 1 km spatial resolution while building a composite hemispheric view from lower resolution images.
As presently recognized, the system proposed by Hughes was deficient in both its camera resources and communication systems infrastructure with regard to the following three attributes. The system proposed by Hughes did not provide real-time images as a result of the delay between frames. Another deficiency was that real-time images cannot be distributed in real time, due to the interval between frames and the slow data rate, as well as the single point data reception and distribution facility. Furthermore, the system proposed by Hughes was deficient in its inability to provide hemispheric (full disk images) in real time. This limitation is due to the limited telemetry channel capacity, limited camera design and the time required to create a composite full disk image. Accordingly, the system proposed by Hughes neither appreciated the significance of providing an infrastructure that would be able to provide real-time images, distribute the real-time images, and provide for the compilation of a composite full disk images in real time.
In 1995, the MITRE Corporation published a study that was performed in 1993. The study examined the use of parasitic instruments on commercial communications satellites for the dual purpose of augmenting government weather satellites and providing a mechanism for low cost test and development of advanced government environmental monitoring systems. This study examined in some detail the application of newly developed megapixel, two-dimensional, CCD arrays to GEOstationary imaging systems and concluded that considerable gains in capacity could be achieved used the CCD arrays. Although the advent of CCD arrays as large as 4096×4096 was anticipated at the time the study was performed, the authors recognized that an array of 1024×1024 was the largest practical size available for application at that time.
Two distinct types of CCD array applications were considered, time delay integration (TDI) and “step-stare”, as alternatives to the traditional “spin-scan” or “flying-spot” imaging techniques. The TDI approach can be viewed as a modification of the “flying-spot” in that it uses an asymmetrical two-dimensional array, e.g., 128×1024, oriented with the long axis vertical so as to reduce the number of East-West scans. In this technique, every geographic scene element is sampled 128 times, which increases the signal-to-noise level. However, communication satellites are relatively unstable platforms. With a single pixel integration time on the order of milliseconds, spacecraft movement during the accumulation of over 100 samples may degrade the spatial resolution within any one scene element. This effect, which is in addition to the navigation and registration degradation due to scan line shift, is called “pixel spread”. Image spread over long integration periods also degrades or precludes low illumination or night observing at visible wavelengths.
The “step-stare” approach was identified in the MITRE study as being the preferred technique. A large, two-dimensional CCD array in this technique is used to capture a portion of the image of the earth. The optical pointing is incrementally “stepped” across the face of the earth by an amount nearly equal to its field of regard at each step. The overlap ensures navigational continuity and registration correctness. With reasonable, but not extraordinary, satellite stability, the frame time may be increased to milliseconds so as to achieve required levels of sensitivity with compromising navigational or registration criteria or image quality.
The MITRE study proposes the use of sub-megapixel arrays (1024×512). With a dwell time per frame of approximately 150 milliseconds, an entire composite full earth disk image at 500-meter spatial resolution could be created from a mosaic of nearly 1,200 frames in relatively few minutes. The maximum exposure time to create an image in daylight is much shorter than 150 milliseconds for most CCD arrays. Furthermore, a reasonably stable satellite undergoes little motion during such a brief time interval thus reducing pixel spread. In order to ensure coverage of the entire earth's surface, frames are overlapped by an amount defined by the satellite stability. This step-stare technique steps the frames in a line from North to South or from East to West, simultaneously exposing all pixels in an array. This ensures accurate registration and navigation of image pixels.
According to the MITRE study, the time between frames in a 500-meter resolution mosaic image of the earth is three minutes (equal to the time needed to create the mosaic). As presently recognized, during this three-minute interval the motion of objects observed, such as clouds and smoke plumes, will cause the object's apparent shape to change in a discontinuous fashion. The continuity of successive observations will thus be compromised and degrade “seamless” coverage by an amount proportional to the velocities of the objects causing the shapes to apparently change. This degradation is called image smear and becomes more apparent as the time between frames increases image smear, thus putting a premium on decreasing the time to create a mosaic of the full disk image.
As presently recognized, with sufficient stability it is possible for a CCD imaging system to allow the shutter to remain open to collect more light to enhance low illumination performance. The impact of CCD arrays in a step-stare scan on night imaging is not noted in the MITRE study. Low illumination imaging is possible by reducing the stepping rate and allowing the camera field to dwell on the area of regard for a predetermined amount of time while integrating its emitted light. At the time of the MITRE study, time exposures to achieve night imaging capability would have increased the time to acquire a full disk image of the earth to about 24 minutes, or about the same amount of time as the flying spot technique. Furthermore, the significance of obtaining real-time night images or the mechanisms needed to obtain the images was never appreciated and thus not realized. In the MITRE study, data distribution was accomplished either by embedding a low data rate in the spacecraft telemetry or directly to receive sites by preempting the use of one of the satellite's transponders. While the emphasis was on rapid full disk imaging, no special considerations were given to disseminate the data either live or globally.
In 1995 the Goddard Space Flight Center announced a study called the “GEO Synchronous Advanced Technology Environmental System” (GATES) that was expected to lead the development of a small satellite system equipped with a “push broom” scanning linear CCD array imaging device. This system was to use motion induced by the satellite's attitude control system to make successive scans of the visible earth's disk. The satellite's attitude control momentum wheels would be used to slew the entire system back and forth 12 times while the field of regard of the camera's linear array is stepped from North to South to achieve a full disk scan in about 1o minutes. This system uses a 1,024 pixel long one-dimensional linear CCD array “flying spot” similar to, but much longer than, the GOES eight pixel array.
As presently recognized, limitations of the GATES system are that neither live images nor night imaging is possible. Data was distributed from a single receive site via the Internet. A limitation common to the Hughes proposed system, the MITRE system and the GATES system is that none of the systems appreciate the interrelationship between providing a real-time continuous monitoring capability of the entire earth that is accessible from a geostationary earth orbit while providing high-resolution images. In part, the limitation with all of the devices is that none of the devices would be able to reliably provide the “watchdog” high resolution imaging function that would provide a remote user with valuable real-time data of dynamic situations occurring at or near the earth's surface.
In U.S. Pat. No. 4,688,091 to Kamel et al, filed May 6, 1986, issued Aug. 18, 1987, there is disclosed a system for achieving spacecraft camera image registration comprising a portion external to the spacecraft and an image motion compensation system (IMCS) portion onboard the spacecraft. Within the IMCS, a computer calculates an image registration compensation signal that is sent to the scan control loops of the onboard cameras. At the location external to the spacecraft, the long-term orbital and attitude perturbations on the spacecraft are modeled. Coefficients from this model are periodically sent to the onboard computer by means of a command unit. The coefficients take into account observations of stars and landmarks made by the spacecraft camera themselves. The computer takes as inputs the updated coefficients plus synchronization information indicating the mirror position of each of the spacecraft cameras, operating mode, and starting and stopping status of the scan lines generated by these cameras, and generates in response thereto the image registration compensation signal. The sources of periodic thermal errors on the spacecraft are discussed. The system is checked by calculating “measurement residuals”, the difference between the landmark and star locations predicted at the external location and the landmark and star locations as measured by the spacecraft cameras.
Therein it is also disclosed that U.S. Pat. No. 3,952,151 discloses a method and apparatus for stabilizing an image produced by, for example, a camera on board a satellite by sensing the instantaneous attitude displacement of the satellite and using these signals to adjust the image generating beam at the ground station.
Thereafter there are disclosed secondary patent references, U.S. Pat. Nos. 3,223,777, 3,676,581, 3,716,669, 3,769,710, 3,859,460, 4,012,018, and 4,300,159.
In U.S. Pat. No. 4,688,092 to Kamel et al, filed May 6, 1986, issued Aug. 18, 1987, there is disclosed that pixels within a satellite camera image are precisely located in terms of latitude and longitude on a celestial body, such as the earth, being imaged. A computer on the earth generated models of the satellite's orbit and attitude, respectively. The orbit model is generated from measurements of stars and landmarks taken by the camera and by range data. The orbit model is an expression of the satellite's latitude and longitude at the subsatellite point, and of the altitude of the satellite, as a function of time, using as coefficients the six Keplerian elements at epoch. The attitude model is based upon star measurements taken by each camera. The attitude model is a set of expressions for the deviations in a set of mutually orthogonal reference optical axes as a function of time, for each camera. Measured data is fit into the models using a walking least squares fit algorithm. A transformation computer transforms pixel coordinates as telemetered by the camera into earth latitude and longitude coordinates using the orbit and attitude models.
In U.S. Pat. No. 5,963,166 to Kamel, filed Jul. 23, 1998, issued Oct. 5, 1999, there is disclosed a precise spacecraft camera image navigation and registration system and method wherein a computer on board the spacecraft and a ground system comprising at least two ground stations precisely compute image navigation and registration data from precise data measurements. The computer on board the spacecraft (spacecraft control system) uses precise star tracker, gyro, and earth sensor attitude data to precisely point the spacecraft and the camera. The ground system utilizes precise star measurement data from the camera and range data from the ground stations time tagged with GPS precise clock data. The ground system uses these precise measurements to determine precise orbit and attitude coefficients and uploads these coefficients to the spacecraft. The computer on board the spacecraft uses these precise coefficients to generate and apply precise signals to compensate for slow orbit and attitude variations and register camera images in real time. The computer on board the spacecraft is also used to generate camera commands to eliminate the need to upload a large number of daily ground commands, which therefore simplifies ground operations.
In U.S. Pat. No. 6,023,291 to Kamel et al, filed Sep. 29, 1997, issued Feb. 8, 2000, there is disclosed a method and system for imaging a celestial object, typically the Earth, with a spacecraft orbiting the celestial object. The method includes steps of (a) operating an imager instrument aboard the spacecraft to generate data representing an image of the celestial object; (b) processing the image data to derive the location of at least one predetermined landmark in the image and a location of edges of the celestial object in the image; and (c) further processing the detected locations to obtain the attitude of the imager instrument. The method includes a further step of outputting the image and the imager instrument attitude to at least one end-user of the image, and/or using the imager instrument attitude to revise the image before outputting the image to the at least one end-user of the image. The generated data preferably represents a one half frame image, and the steps of processing and further processing thus occur at a one half frame rate. The step of processing includes a step of applying the a priori knowledge of the attitude coefficients in processing new observations to determine the imager current attitude.
In U.S. Pat. No. 6,271,877 to LeCompte, filed Jun. 25, 1999, issued Aug. 7, 2001, there is disclosed a system, method and apparatus for collecting and distributing real-time, high resolution images of the earth from GEO include an electro-optical sensor based on multi-megapixel two-dimensional charge coupled device (CCD) arrays mounted on a geostationary platform. At least four, three-axis stabilized satellites in geostationary earth orbit (GEO) provide worldwide coverage, excluding the poles. Image data that is collected at approximately one frame/sec is broadcast over high-capacity communication links (roughly 15 MHZ bandwidth) providing real-time global coverage of the earth at sub-kilometer resolutions directly to end users. This data may be distributed globally from each satellite through a system of space and ground telecommunication links. Each satellite carries at least two electro-optical imaging systems that operate at visible wavelengths so as to provide uninterrupted views of the earth's full disk and coverage at sub-kilometer spatial resolutions of most or selected portions of the earth's surface.
In U.S. Pat. No. 6,331,870 to LeCompte, filed May 21, 2001, issued Dec. 18, 2001, there is disclosed a system, method and apparatus for collecting and distributing real-time, high resolution images of the earth from GEO include an electro-optical sensor based on multi-megapixel two-dimensional charge coupled device (CCD) arrays mounted on a geostationary platform. At least four, three-axis stabilized satellites in geostationary earth orbit (GEO) provide worldwide coverage, excluding the poles. Image data that is collected at approximately one frame/sec is broadcast over high-capacity communication links (roughly 15 MHZ bandwidth) providing real-time global coverage of the earth at sub-kilometer resolutions directly to end users. This data may be distributed globally from each satellite through a system of space and ground telecommunication links. Each satellite carries at least two electro-optical imaging systems that operate at visible wavelengths so as to provide uninterrupted views of the earth's full disk and coverage at sub-kilometer spatial resolutions of most or selected portions of the earth's surface.
In U.S. Pat. No. 6,463,366 to Kinashi et al, filed Mar. 12, 2001, issued Oct. 8, 2002, there is disclosed an attitude determination and alignment method and system use electro-optical sensors and global navigation satellites to determine attitude knowledge for a spacecraft, satellite or a high-altitude aircraft. An onboard inertial navigation system uses global navigation satellite system equipment and an attitude determination system uses an electro-optical sensor. The electro-optical sensor views the navigation satellites as surrogate stellar reference sources. The electro-optical sensor replaces the function of a star sensor or tracker and associated processing required for an onboard attitude determination system. Navigation and timing information generated by the GPS/GNSS-INS is used to perform required attitude determination system functions.
In U.S. Pat. No. 6,504,570 to LeCompte, filed Nov. 26, 2001, issued Jan. 7, 2003, there is disclosed a system, method and apparatus for collecting and distributing real-time, high resolution images of the earth from GEO include an electro-optical sensor based on multi-megapixel two-dimensional charge coupled device (CCD) arrays mounted on a geostationary platform. At least four three-axis stabilized satellites in geostationary earth orbit (GEO) provide worldwide coverage, excluding the poles. Image data that is collected at approximately one frame/sec is broadcast over high-capacity communication links (roughly 15 MHZ bandwidth) providing real-time global coverage of the earth at sub-kilometer resolutions directly to end users. This data may be distributed globally from each satellite through a system of space and ground telecommunication links. Each satellite carries at least two electro-optical imaging systems that operate at visible wavelengths so as to provide uninterrupted views of the earth's full disk and coverage at sub-kilometer spatial resolutions of most or selected portions of the earth's surface.
There is therefore a demonstrated need to provide improved INR systems that are devoid of the above-noted deficiencies.